1. Field of the Invention
This invention relates to alloys and more particularly, to aluminium alloy sheets for discs having good platability.
2. Description of the Prior Art
As is known in the art, substrates for discs such as magnetic discs, optical discs and optical-magnetic discs and the like should be non-magnetic and have high rigidity sufficient to withstand the rotation of high speed and good resistance to corrosion. In view of the above, it is conventional to use aluminium alloys as the substrate. As described above, several types of discs are known and substrates for magnetic discs will be described herein only for convenience's sake.
Because the distance between a substrate for magnetic disc and a magnetic head is so small as less than about 1 .mu.m and the disc is rotated at high speed relative to the head, the smoothness of the substrate for the disc is also one of important characteristics.
In recent years, the magnetic recording density is so increased that the distance between the disc substrate and the magnetic head becomes much smaller with an attendant smaller unit recording area (i.e. bit size). This in turn requires that the substrate surface have a roughness as small as possible. In addition, it is also required that the defects on the substrate surface be as small as possible not only in size, but also in number.
In order to make a smooth substrate for a magnetic disc, there has been proposed a method in which an aluminium alloy substrate is subjected to anodization or plating to form a hard film on the substrate and is then polished.
Typical aluminium alloys for magnetic discs which have been used for plating are A,A5086 alloys. JIS 7075 alloys are sometimes used for these purposes.
However, these known alloy materials have the drawback that they tend to be roughened on the surface thereof due to the fact that the crystallization phase (Al--Fe, Al--Mn--Fe and the like) or the precipitation phase (Al--Cu--Mg in the JIS 7075 alloys) of an aluminium alloy sheet comes off at the time of polishing or comes of by dissolution at the time of pretreatments for plating.
Fabrication of discs from JIS 7075 alloy, which are heat treatable alloy, by punching or cutting from a rolled sheet of the alloy involves the drawback that where the disc is annealed to remove the strain therefrom, the cooling speed must be properly controlled to suppress the internal stress.
As described above, for the reasons that the aluminium alloy disc tends to be roughened on the surface thereof, and pits (small holes) are liable to be produced on the plating layer owing to the roughening, it is the usual practice for the known materials that the plating film is formed in a relatively large thickness of about 30 to 50 .mu.m and is subsequently polished.
However, in order to improve the productivity and reduce the cost, it is important to make the plating layer with a small thickness. Aside from the thickness of the plating film, it is also important to reduce a number of pits and to reduce the roughness in the pretreatment. To this end, attempts were made to use 99.9 wt % or 99.99 wt % Al metal for producing fine intermetalic compounds. However, the mere increase of the purity in the metal results not only in an increasing roughness on the surface being plated, but also in a lowering of the plate adhesion.